Devices which convert electrical energy to mechanical energy generally convert only part of the electrical energy into useful work. The rest of the input electrical energy is converted into other forms. Some energy may be lost to friction as heat. Some energy may be stored as potential energy in electric or magnetic fields, or potential energy of compressed springs.
Solenoids designed to provide fast turn off by means of mechanical springs store significant energy in spring compression and in magnetic fields. Solenoid drive systems which control these solenoids must handle the stored energy when the solenoid shuts off. Some drive systems simply dissipate this energy as heat. These systems are very energy inefficient, and they suffer from the problems associated with disposing of the generated heat. Such systems must accept high thermal stresses, and they inefficiently use semiconductors as heat dissipators in linear operation. Such systems need large heat sinks, and have reliability problems due to high semiconductor junction temperatures.
A schematic diagram of a contemporary solenoid driver is shown in FIG. 1. Typically, the solenoid 101 has an inductive coil with inductance L.sub.s and resistance R.sub.s, an iron piston, and a spring. The iron piston is located so that when there is sufficient current running through the coil, a magnetic field generated by the coil draws the iron piston into the coil. The spring connects to the piston and to a fixed support so that when the iron piston is drawn into the coil the spring is either stretched or compressed. To turn on the solenoid 101, a voltage is applied to the gate of transistor 102 so that the transistor 102 allows current to flow through the solenoid 101 and the transistor 102. During turn on, energy is stored in the magnetic field and the mechanical spring.
When the power to a solenoid 101 is turned off, the inductance in the solenoid does not let the current through the solenoid 101 fall immediately to zero. Instead the solenoid 101 generates a voltage to oppose the change in current. This voltage is equal to the time derivative of the current times the inductance. As the solenoid 101 shuts down, stored energy provides the power to maintain a current.
If the current is cut off quickly the time derivative is large and the voltage generated in the solenoid 101 can drive a current through the circuit. Heat energy is dissipated in the power transistor 102 during turn off by resistive heating.
In a typical solenoid system of this type which cycles solenoid power on and off at a rate of 90 Hz, approximately 60% to 80% of the power dissipation is dissipation of stored energy. Heat sink size and power transistor size are therefore mostly determined by the need to dissipate stored energy, and technology advances aimed at improving transistor energy efficiency for a given device size are largely irrelevant.
The prior art half bridge architecture shown in FIG. 2 solves the problem of energy dissipation in the power transistor by providing a path to return stored energy to the power source. Two transistors 202 and 203 and two diodes 206 and 207 are used to turn a solenoid 201 on or off.
FIG. 3 shows the operation of the half bridge circuit of FIG. 2 when the solenoid is being turned on. In FIG. 3, the predominant current flows through the transistor 303, the solenoid 301, and transistor 302. The solenoid 301 is driven from zero current to its steady state current with the exponential rise characteristic of current for inductive and resistive circuits (LR circuits). FIG. 4 shows the characteristic current rise in a solenoid circuit when a constant voltage is applied.
FIG. 5 shows the operation of the same half bridge during turn off of the solenoid 501. With both transistors 502 and 503 off, the current decreases, causing a voltage across the solenoid 501. The induced voltage continues to drive the current in the original direction, through the solenoid 501. This current can most easily flow through the two diodes 504 and 505 as shown in FIG. 5. The path through the transistors 502 and 503 is blocked when the transistors are off. As seen from the current path I.sub.L in FIG. 5, the current flows from the ground 506, through diode 504, through the solenoid 501, and through the diode 505 to the positive terminal 507 of a power supply. The power source polarity is effectively reversed from when the transistors are on, so the power source forces a rapid decay of current. Current is also forced through the power supply thereby returning energy to the power supply.
FIG. 6 shows the effect a change in polarity has on current in an LR circuit. Without the diodes, a power supply with reversed polarity would eventually turn the current around and force a current of equal steady state magnitude in the opposite of the original direction.
The current direction is not reversed in the half bridge circuit of FIGS. 2, 3, and 5 because the diodes 505 and 504 go to a high impedance state when reverse voltage biased (at zero current). Current through the solenoid 501 stops, and the solenoid is not reactivated by a reversed current.
A disadvantage of the prior art half bridge architecture when employed for driving solenoids is the cost of circuit components, diodes and transistors. The prior art half bridge requires a minimum of four circuit components per solenoid. N solenoids require 4*N components. A system with fewer components can be cheaper.
Another problem with current solenoid control circuits is solenoid design. DC circuit solenoid design is restricted by conflicting requirements on solenoid wire resistance. The resistance must be high to control current and minimize resistive heating in the solenoid. When the steady state current is reached, power generation in the solenoid=V.sup.2 /R. The resistance of the solenoid needs to be high so that the solenoid will not over heat when driven by the highest anticipated voltage. However, for fast activation of the solenoid the ratio L/R should be high. With a fixed inductance fast activation requires lower resistances. With solenoid drivers as configured in FIG. 1 a compromise must be chosen to accommodate both goals.